A wide variety of pneumatic fluid dispensers that dispense adhesives, sealants, lubricants and other fluids and liquids in a wide range of viscosities are well known. Pneumatic fluid dispensers have historically been favored because, in a manual dispenser, they are light and easy to manipulate, as well as being relatively inexpensive to manufacture and operate. Further, pneumatic technology has continued to improve, so that pneumatic fluid dispensers continue to be widely used. However, applications for faster and more precise fluid dispensing both manually, and on a production line, continue to grow rapidly; and the requirements and specifications for fluid dispensing applications are ever more rigorous. Many applications require that fluids be dispensed in very precise volumes and at very precise locations. Further, many fluids experience viscosity changes during a dispensing process. Such stringent application demands are pushing the capabilities of pneumatic fluid dispensers.
Pneumatic fluid dispensers commonly apply a pressurized fluid, for example, compressed air or shop air commonly found in a manufacturing area. Upon a manually initiated, or automatically generated, command signal, the compressed air is applied to, and pushes against, a piston in a barrel reservoir or syringe holding a fluid. The compressed air is applied at a fixed pressure over a controllable and variable time duration. Thus, the volume of fluid dispensed is proportional to the time that the compressed air is applied to the piston. In many applications, the fluid being dispensed has a relatively constant viscosity, and the volume being dispensed with each dispensing operation is fixed. In those applications, a pneumatic dispenser can be operated to accurately and reliably dispense the desired volumes of fluid.
However, in other applications, the viscosity of the fluid may change from one fluid dispensing cycle to another. In such applications, applying the compressed air for fixed time periods over successive dispensing cycles will result in different quantities of fluid being dispensed as the viscosity changes. Further, the compressibility of air makes it difficult for a pneumatic fluid dispensing system to consistently and precisely dispense a predicable fluid volume as the barrel reservoir moves from full to empty. Further, it is increasingly difficult for pneumatic fluid dispensing systems to meet dispensing volumetric accuracy and repeatability specifications of newer fluid dispensing fluids and applications.
It is also known to use an electromechanical drive system to move a piston in a barrel reservoir of a dispensing system. Such an electromechanical system imparts a positive displacement to the piston in the barrel reservoir and thus, provides a fluid dispenser having a volumetric accuracy and repeatability often superior to that of a pneumatic fluid dispensing system. With many hand held dispensers, a flexible cable transfers a mechanical force from an electric motor located on a bench-top to a screw or other mechanical drive in the hand held dispenser. With known electromechanical piston-drive systems, the electric motor must be sufficiently large to apply forces necessary to accurately move the piston over a wide range of fluid viscosities. Thus, such electromechanical drive systems are often larger and more complex and expensive than known pneumatic fluid dispensers.
Therefore, there is a need for an improved pneumatic fluid dispensing system and method that does not have the disadvantages discussed above.